Permanent magnet electrodynamic transducer



May 15, 1951 A. l.. si-:IFRIED 2,553,150

PERMANENT MAGNET ELECTRODYNAMIC TRANSDUCER Filed omi. 28, 19497/IIIIIIIIIII/7 fais ATTORNEY Patented May I', 1951 UNITED STATES PATENTOFFICE PERMANENT MAGNET ELECTRODYNAMIC TRAN SDUCER Albert L. Seifried,Millington', NQJ., assigner to Thomas-ArEdison, Incorporated, WestOrange,- N. J., a corporation of New Jersey Application `October 28,1949, Serial No. 124,041

4 Claims. l

This invention relates to a small electroacoustical transducer forphonographs adapted for reproducing speech recordations, and moreparticularly thel invention relates to an improved transducer' which isadapted to feed into a small soundtube leading to anacoustic-typeheadset.

In'thereproduction of recorded speech, acoustic receivers are commonlyused which are Worn on'the head and adapted to't over or into the ears,a pair of such receivers being herein referred to as a headset. Suchacoustic headset ispreierred over a headset of the electroacous tic'type-since it is ftypically smaller f and lighter. However, heretoforethe acoustic-type headset has been cumbersome because of the need for afairly' largeandstiff acoustic tube coupling thel headset to` thereproducer. The present transducer is-adapted to- Work' into a verysmall,

light?l coupling tube-Which Will not encumber to Another object is toprovide such a transducer` which vhaseflicient high-frequencyV response'to enable theV faithful reproduction abovementioned.

Other objects and features of the invention will be apparent from thefollowing description and the appended claims.

In'the description of my. invention reference-` is had to theaccompanying drawings, of which:

Figure 1 is aside elevational View of an electroacoustical transducer,with amounting bracketgin Which my invention is embodied;

Figure-2 isv a` front ,elevational view `of kthis transducer; A

Figure 3 isfa sectiontotenlar-ged scalel taken substantial-1y onv theline'B-'bi Figure 2;

Figure 4 is a backV ViewY of the-.transducer asv seen-from the llinea- 4of Figure 3;

Figure 5 is a section taken` substantially on the line 5;-5 of Figure 3;

`Figure Gris a View to enlarged scale of the diaphragm assembly; and

Figure 'l is a fractional sectional view to enlarged scale taken throughthe axis of the transducerto show details'of the vibratory system of thetransducer.

The present transducer includes a diaphragm' the essential parts of thedevice in concentric For instance, positionedwithin'the holder I is athin'circular'diaphragm` i5 the peripheral edge of which contacts' the:

relation to each other.

inner surface vci this locatingl'flange. A singlelayer voice coil i6 isconnected to the side-of the diaphragm which faces away from the holderI0. This coil is located in concentric relation tofthe peripheral edgeof the diaphragm and is therefore located in concentric relation to theflange i2.. The-diaphragm is spaced from the Wall il by a thin annulargasket Il'. Another such gasket is positicnedagainst the outer side ofthe diaphragm and positioned-flat against this gasket is or similarelements, spaced at caudal-intervals` about the axis of the device.These rivets are positioned near the'rirnL flange through the disk I8,thegasket I'I, the diaphragm :I5 and the Wall I to clamp these partstightly together.v An advantage of securing these parts intoa permanent`unitary subass'emblyis that the delicate-diaphragm is protectedlargelyby the disk and the holder 'to permit stocking,

andhandling -of thesubassembly without likelihood of damasethereto.

In the inner side of the annular diskl I8 there are tvvo approximatelyparallel grooves 20 running from the inner to the outer edges of thedisk (Figures 5 and 7). These grooves form clearanceopenings between thedisk and the adjacent gasket II for lead-out rwires 2l of the voice coilri, the portion of thelead-out Wires adjacent to thedisk beingscovered`byinsulatingv These lead-out f sleeves 22 (Figures 5 rardz).

I 2 and pass- Wires and sleeves pass through clearance slots 23 providedin the rim flange I2 and the wires are connected to a pair of terminals24 anchored to an insulating plate 25. This plate is secured to thediaphragm holder as by an elongated one of the aforementioned rivetsindicated in Figure 3 as I 9a.

The magnetic system of the transducer also comprises a member ofmagnetically soft material in the form of a cup 26. This cup has anoutwardly-extending rim flange 2l which seats on the outer border of theannular disk I8 and the peripheral edge of which fits into the fiange I2to locate the cup also in concentric relation with the voice coil.Mounted in this cup at the axis thereof is a cylindrical core 28 whichis staked or soldered to the bottom wall of the cup at 29. The outer endof this core ts within the voice coil at a clearance spacing therefrom.Since the core is positioned'at the axis of the cup and the latter ispositioned in concentric relation with the voice coil, the core istherefore also concentric with the coil. In the preferred construction,the core is made of permanent-magnet material as of one of the alloys ofaluminum, nickel, cobalt and iron known as Alnico, in which case theouter end of the core is preferably provided with a soft iron pole piece28a. However, as an alternative construction, it will be understood thatthe core may be made of magnetically soft iron and may be ener'l gizedby a suitable eld coil not herein necessary to show.

The cup 26 is secured in assembled relation to the aforementionedsubassembly by four rivets 3D, or like elements, which are interposedbetween the rivets I9. These rivets 30 pass through the flange 21 andthe parts within the holder I in the manner of the rivets I9. In orderthat the rim flange 2l of the cup may lie flat against the disk I8 forclose magnetic coupling therewith, it is provided with four notches 3!to clear the heads of the rivets ES (Figure l).

A resilient mounting for the present transducer is preferred andcomprises a rubber ring 32 placed around the magnetic cup and heldthereon by a circular plate 33 secured as by spot welding to the backside of the cup. In this ring there is a peripheral groove 34 receivedby an apertured leg 35 of a mounting bracket 36 which is held to asuitable support 31 as by screws 38 (Figure l). The plate 33 is weldedto the cup before the latter is secured to the holder. To provide easyaccess to the rivets 3@ this plate is provided with four respectiveClearance notches 33a.

The sensitivity of the present instrument is dependent, among otherfactors, upon the magnetic flux density in the air gap between the core28 and the annular disk I8. In order that the gap density@ may be amaximum for a permanent magnet of given strength, the magnetic cup ismade suioiently large in diameter to minimize leakage losses.

In order that good high-frequency response is obtained the Vibratorysystem-i. e., the diaphragm and voice coil-is designed to be resonant atthe highest frequency to be reproduced. In this design, the diaphragmmass is kept low by choosing a thin diaphragm of light metal, say of.002 thick aluminum, and by cementing the coil directly to the diaphragmWithout use of a coil frame; further, the stiffness of the diaphragm ismade high and break-up of the diaphragm at high frequencies is preventedby 4 offsetting the central portion of the diaphragm at 39 and by usinga diaphragm of effectively small diameter.

Novel features of the present invention whereby in a simple designleakage flux is minimized, the resonant frequency of the diaphragm ismade high, and parts are accurately positioned relative to each otherlie in the use 0f a large diaphragm and large magnetic cup equal indiameter to each other so that they can be centered by the commonlocating flange I3, and in the use of gaskets clamped against the sides`of the diaphragm which have a relatively small central opening toreduce suitably the effective diameter of the diaphragm.

A further feature of the present invention lies in the use of simpleparts having essential locating surfaces in concentric relation to eachother so that these surfaces can be established with a high degree ofaccuracy, and in the use of a construction wherein these parts arelocated accurately in relation to each other by one common locatingsurface, which is the inner peripheral surface of the locating flangeI3.

The preferred embodiment of my invention herein shown and described isintended t0 be illustrative and not necessarily limitative of theinvention, since this embodiment is subject to changes and modificationswithout departure from the scope of my invention, which I endeavor toexpress according t0 the following claims.

I claim:

1. An electroacoustioal transducer comprising a circular diaphragm, acircular diaphragm holder having a peripheral flange engaging theperipheral edge of the daiphragm and having a central opening for theemission of sound from the inner side of the diaphragm, a voice coilmounted on the outer side of the diaphragm in concentric relation withthe peripheral edge thereof, an annular disk of soft magnetic materialin juxtaposition with the outer side of said diaphragm and in peripheralengagement with said flange with the inner peripheral edge of said diskconfronting the outer side of said coil in spaced relation thereto, anannular gasket between one side of the diaphragm and the inner wall ofsaid holder and another annular gasket between the outer side of thediaphragm and said magnetic disk to space the diaphragm from the diskand holder for free vibration of the central portion thereof, clampingelements spaced at intervals about the axis of said holder near the rimthereof and traversing the holder, gaskets, diaphragm and disk to securethe same into a unitary subassembly, a cup-shaped member of softmagnetic material having an out wardly-extending peripheral flangeseated on said disk and engaging the inner peripheral surface of saidflange of said holder, said flange of said cup having portions cut awayto clear the ange of said clamping elements, a central magnetic coremounted in said cup having the end portion thereof within said voicecoil at a clearance spacing therefrom, and other clamping elementsinterposed between said rstmentioned clamping elements and securing saidperipheral flange of said cup in mounted relation to said subassembly.

2. rIhe combination set forth in claim l Wherein said diaphragm holderhas a at inner wall and wherein the diameters of the central openings ofsaid gaskets are a minor fraction of the diameter of said diaphragm toprovide a cylindrical air chamber between the diaphragm and holderhaving a diameter less than half the overall diameter of the diaphragmand having a length equal to the thickness of one of said gaskets.

3. An electroacoustical transducer comprising a circular diaphragm, acircular diaphragm holder having a rim flange fitting the peripheraledge of said diaphragm and having a central opening for the emission ofsound from the inner side of the diaphragm, a voice coil mounted on theouter side of said diaphragm in concentric relation therewith, anannular magnetic disk mounted in said holder in spaced relation to theouter side of said diaphragm and in peripheral engagement with said rimflange with the inner periphery of said disk confronting the outerperiphery of said coil at a clearance spacing therefrom, a magneticcup-shaped member having an outwardly-extending rim ange seated on saidmagnetic disk and tting said first rim flange to locate the cup inconcentric relation with said holder, a magnetic member mounted in saidcup and having a circular end portion concentric with the cup anddisposed within said voice coil at a clearance spacing therefrom,elements at the rim of said diaphragm holder securing said magnetic diskand diaphragm to said holder to form a permanent subassembly of saidtransducer, and other elements interposed between said rstmentionedelements and securing said magnetic cup in mounted relation to saidsubassembly.

4. An electroacoustical transducer comprising a circular diaphragm, acircular diaphragm holder having a rim flange fitting the peripheraledge of said diaphragm and having a central opening for the emission ofsound from the inner side of the diaphragm, a Voice coil mounted on theouter side of said diaphragm in concentric relation therewith, anannular magnetic disk mounted in said holder in spaced relation to theouter side of said diaphragm and in peripheral engagement with said rimflange with the inner periphery of said disk confronting the outerperiphery of said coil at a clearance spacing therefrom, a magneticcup-shaped member having an outwardly-extending rim flange seated onsaid magnetic disk and fitting said iirst rim :dange to locate the cupin concentric relation with said holder, a magnetic member mounted insaid cup and having a circular end portion concentric with the cup anddisposed within said voice coil at a clearance spacing therefrom,clamping elements traversing said magnetic disk, diaphragm and holder atthe rim of the latter to secure the same into one subassembly, and otherclamping elements between said first-mentioned elements and traversingsaid holder, diaphragm, disk and flange of said cup to secure the cup tosaid subassernbly, wherein said flange of said cup has portions thereofcut away to clear the flange of said first-mentioned clamping elements.

ALBERT L. SEIFRIED.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,915,358 Giles June 2'7, 19332,025,127 Rowe 1 Dec. 24, 1935 2,440,439 Gilman Apr. 27, 1948 FOREIGNPATENTS Number Country Date 460,064 Great Britain Jan. 20, 1937

